The present invention relates generally to Magnetic Resonance Imaging (MRI) systems, and more particularly, to an improved method and system for generating a uniform magnetic field in an open architecture imaging volume.
A typical MRI system having an open architecture is located within an unshielded MRI room and includes a static magnet structure having a fore section and an aft section. A cryostat having a pair of toroidally shaped vessels containing a superconducting magnet are included within the magnetic structure for cooling the superconducting magnet. The vessels are attached by steel spacers and define a patient volume. The length of the spacers corresponds with an amount of available space for a physician to treat a patient. The patient lies on a table that extends within the patient volume.
As a part of a typical MRI, radio frequency (RF) signals of suitable frequencies are transmitted into the patient volume, via RF transmit coils. The superconducting magnet generates a static magnet field for polarizing the hydrogen atoms of the patient. Nuclear magnetic resonance (nMR) responsive RF signals are emitted by the polarized protons, and received from the imaging volume by RF receiver coils. Information encoded within the frequency and phase parameters of the received RF signals, by the use of a RF circuit, is processed to form visual images. These visual images represent the distribution of nMR nuclei within a cross-section or volume of the patient within the imaging volume.
Magnetic Resonance (MR) imaging requires a highly uniform magnetic field to generate good quality images. To increase imaging quality it is desirable to increase field strength of the magnet field. By increasing magnetic field strength stray MR field increases. Thus, a field strength limitation arises when maintaining stray MR field below a specified level to not effect individuals outside of the MRI room. To shield a MRI room from a surrounding environment is sometimes costly and impractical.
Referring now to FIG. 1, a cross-sectional area plot of a superconducting magnet 6 and imaging volume 8 for a typical 0.5 Tesla MRI system having an open architecture, is shown. The vertical axis is R in centimeters, and the horizontal axis is Z also in centimeters. The rectangular areas 10 represent the cryostat. The positive areas 12 correspond to positive cross-sectional areas of the superconducting coils, which are directly proportional to electromagnetic forces generated therein. The negative areas 14 correspond to negative cross-sectional areas of the superconducting coils, which are directly proportional to electromagnetic forces generated therein. A typical 0.5 Tesla open superconducting magnet, as the one in this example, has approximately 100,000 cubic centimeters of superconductor. This MRI system has a coil to coil gap of 60 cm, which yields a room temperature gap of approximately 50 cm for physician access. It is known that a 55 cm room temperature gap is the minimum acceptable for interventional procedures, thus prior art MRI system design does not satisfy the minimal acceptable gap requirement.
Referring now to FIG. 2, a cross-sectional area plot of a 0.5 Tesla superconducting magnet and imaging volume for the typical MRI system with increased gap for improved physician access and increased superconducting magnet forces, is shown. Increasing coil to coil gap from 60 cm to 76 cm for improved physician access has resulted in increased forces between the superconducting coils. Comparing positive areas 12 and negative areas 14 of FIG. 2 with positive areas 12 and negative areas 14 of FIG. 1, it becomes obvious the difference in superconducting magnet size and electromagnetic forces necessary to generate a magnet with significantly improved physician access for interventional procedures, using the traditional MRI system. The superconducting magnet, of FIG. 2, has approximately 392,000 cubic centimeters of superconductor and relatively increased magnetic forces, which is clearly not feasible to build.
A concept has been suggested to generate and combine a pulsing polarizing field of poor homogeneity and high amplitude with a static readout field of good homogeneity and small amplitude to produce images. The pulsed polarizing field is generated from a resistive polarizing magnet. However, the polarizing field must be pulsed at a high duty cycle rate to achieve acceptable imaging times. The high duty cycle of the polarizing magnet generates large resistive heating that require a large amount of cooling power within the MRI system. Cooling of the MRI system is a limitation with current MRI systems, which further causes this concept to be costly and impractical. Another disadvantage with this concept is that it only has limited pulsing sequence options, which restricts the amount of different medical conditions a physician is able to view.
Additionally, available design configurations of the open architecture MRI system are limited. For example, the amount of space available for a treating physician is constrained by the MRI system physical and operational requirements. Depending upon a treatment being implemented, a physician may desire different amounts of space or different orientations of the space available.
It would therefore be desirable to design an open architecture MRI system that provides potentially increased magnetic field uniformity, that minimizes generated stray field, that provides multiple feasible pulsing sequence options, and multiple design configurations.